1. Field of the Invention
This invention relates to a single-crystal Ni-based super-heat-resistant alloy which has an improved creep rupture strength and creep rupture ductility and which is used mainly as a material of gas turbine engine blades.
2. Description of the Prior Art
In general, rupture of metals at a high temperatures takes place along grain boundaries. It is therefore possible to greatly increase the creep rupture strength of a turbine blade at high temperatures by using a metal of a single-crystal structure having no grain boundaries and by applying a suitable heat treatment to that metal. The following single-crystal Ni-based super-heat-resistant alloys have been developed from this concept: Alloy 444 (disclosed in U.S. Pat. No. 4,116,723), Alloy 454 (disclosed in U.S. Pat. No. 4,209,348) and Alloy 203E (disclosed in U.S. Pat. No. 4,222,794) by United Technologies Corporation; NASAIR100 by Air Research Corporation; and CMSX-2 (disclosed in Japanese Patent Application Laid-Open Publication No. 89451/82) and CMSX-3 (disclosed in Japanese patent Application Laid-Open Publication No. 190342/84) by Canon Muskegon Corporation. The creep rupture strength of each of these single-crystal alloys is remarkably higher than those of conventional polycrystal alloys, but these single-crystal alloys are still unsatisfactory from the standpoints of compositional balance and structure control. It has been found that, the alloy NASAIR100, for example, precipitates the detrimental phases such as .alpha.-W phase, .mu. phase, etc. thereby reducing the creep rupture strength. In order to prevent the precipitation of the detrimental phases such as .alpha.-W phase, etc., it is necessary to reduce the amount of W, Mo, Ta, etc. to be added. However too much reduction in contents of these elements results in a reduced creep rupture strength, since these elements are effective in strengthening the alloy.